Causes of ADHD

October 01st, 2012

ADHD is a controversial disorder despite being recognised as a valid clinical entity for many decades. A recent study (Thapar et al., 2012) has examined the evidence on the causes of ADHD in a review in the Journal of Child Psychology & Psychiatry. The paper is open to all, not just subscribers if you want to read it. At the time of this post it was available in the Early View part of the JCPP homepage. I expect it will move into the next volume of the journal proper at some point in the next 3-months. Here are some highlights for those who don’t want to read the paper.

1st degree relatives of those with ADHD are 2-8 x more likely to also have ADHD than the relatives of people without ADHD.

Many twin studies have reported heritability rates of 71-90%. Note that heritability rates include not only pure genetic influences but gene-environmental interactions. Therefore high heritability rates don’t rule out environmental factors at all.

Five adoption studies in which the similarities of children with ADHD to related and unrelated caregivers have shown high heritability rates. In other words, take a kid with risk of ADHD and place them in an adopted family with little risk and they are still likely to display ADHD.

It’s very important to recognise that you can’t really separate genes and environment and it’s likely that the interaction between both is vital in how all neurodevelopment disorders manifest. For example, genetic risks might lead to ADHD in the presence of environmental factors such as maternal smoking or peer rejection. Or genetically coded risks via child/parent temperament might increase the risk of environmental factors such as maternal smoking or peer rejection.

There is no single gene involved and the effect sizes for single genes are typically small. What is known about genetic factors in ADHD cannot be used in clinical practice to assess risk or help diagnose.

Thapar et al. remind us that the significance of environmental factors, such as maternal smoking, may have been overrated because it is possible that they have their effects via a gene-environment interaction rather than operating as a stand-alone environmental variable. They reported that pre- and peri-natal factors like maternal smoking, alcohol and substance misuse were risk factors but not proven causes. Same goes for lead, pesticides, low birth weight, prematurity and maternal stress. Nutritional deficiencies (e.g., zinc, magnesium) were considered correlates (i.e., they exist together) but have not been proven causal. Same with family adversity and low income and nutritional factors (e.g., high sugar and artificial colourings). Severe early deprivation was considered a likely causal risk factor.